The present invention relates to infrared (IR) detectors characterized in having a compact size and improved cooling efficiency.
Infrared radiation detector devices commonly comprise a casing, an electronic IR sensitive detecting component, hereinafter xe2x80x9cdetector elementxe2x80x9d, and an optical window such as a lens or an infrared transmitting means. Furthermore, said devices comprise a cryogenic cooler. By the term xe2x80x9ccryogenic coolerxe2x80x9d is meant herein a device which effects cooling by direct Joule-Thomson effect. The fluid used in said device may be called xe2x80x9ccryogenic fluidxe2x80x9d. As is well known, the direct Joule-Thomson is based on the adiabatic expansion of a gas below a characteristic temperature, known as the inversion temperature, which effects cooling of the volume into which the expansion of the gas occurs, while in the reverse Joule-Thomson effect the adiabatic expansion of a gas above its inversion temperature effects heating of the surroundings.
U.S. Pat. No. 4,474,036 describes an IR detector which comprises an envelope in the form of a Dewar in which a vacuum space is present between an outer wall and an inner element, cooled by a cooling element, which may be based on the Joule-Thomson effect.
In prior art IR detector, however, the volume cannot be reduced as desired. The distance from the optical window to the detector element is fixed, based upon the optical design, and the cooling element must have a certain length, to permit an effective pre-cooling.
In some applications the size of the entire IR detector is of extreme importance. More particularly, the compact size of the detector in missile heads is of paramount importance. Due to the design of the missile head, the space allocated to the detector device is quite limited and therefore it is important to provide a detector of compact size. Furthermore, the flight of the missile causes considerable vibration of the detector, which impedes the detector""s performance. A compact size of the detector device tends to provide additional rigidity to the assembly and thus to reduce the vibration of the detector and to improve its performance. Therefore, efforts have been made to design detectors of compact size.
IL 81534 describes an IR detector with a Joule-Thomson cooler comprising a housing, infrared transmitting means at its front, and a Joule-Thomson cooling element wound about a conical core and enclosed in a pre-cooling jacket, of which said core forms one of the surfaces. The cooler element is in a coiled configuration, wherein the inlet is attached to a source of pressurized gas and the outlet, which is fitted with an expansion nozzle, is located in the vicinity of the detector element. The pre-cooling jacket effects cooling of the cryogenic gas while it is flowing through the tube. Thus, gas expanding through the orifice reaches a lower temperature and effects more efficient cooling of the detector.
The spiral configuration of the cooler tube is imperative for achieving the desired pre-cooling effect. However, in spite of the spiraled configuration, which is essential for achieving the desired pre-cooling effect, the detector devices of the prior art have a relatively large volume.
It is a purpose of the present invention to provide an infrared radiation detector device comprising a tube through which a gas flows, hereinafter xe2x80x9ccooler tubexe2x80x9d, of a sufficient length so to achieve the desired pre-cooling effect, the volume of which device is smaller than that of comparable prior art devices.
It is another purpose of this invention to provide a compact infrared radiation detector device with efficient heat exchange between the cooler tube and the discharge gas.
It is a further purpose to provide such a device having a rigid structure, which reduces the detector displacement amplitude.
It is a still further purpose to provide such a device in which the exhaust back pressure of the discharge cooling fluid is reduced, thus lowering the boiling temperature of the cooling fluid.
It is a still further purpose to provide in such a device a sufficient volume for self regulated flow minicooler systems.
Other purposes and advantages of the invention will become clear as the description proceeds.
The IR detector device according to the invention comprises:
a) a casing, having an optical window for admitting radiation;
b) a cold shield having a detector mounted therein and a filter mounted between said optical window and said detector;
c) a cooler tube in coiled configuration, having an inlet in communication with a pressurized gas source and an expansion nozzle located in the vicinity of the detector;
d) a pre-cooling jacket, enclosing the cooler tube and having a discharge gas inlet in the vicinity of the detector and a discharge gas vent to the outside;
e) the cooler tube comprising a first section, beginning at the inlet, a second section, terminating at the expansion nozzle, and a communication between said first and second sections;
f) the pre-cooling jacket comprising a first section which houses the first section of the tube, a second section which houses the second section of the tube, and a communication between said first and second sections;
g) the first section of the jacket bounding an inner space which encloses the second section of the jacket,
whereby the second section of the cooler tube, contained in the second section of the jacket, is also entirely contained in the inner space defined by the first section of the jacket and makes no contribution to the overall volume of the detector device.
By xe2x80x9coptical windowxe2x80x9d is meant, in this description and claims, any component that allows visible or IR radiation to pass through it, e.g. a lens or any IR transmission means.
Preferably, a portion of the cold shield extends into the inner space bounded by the first section of the pre-cooling jacket. Said first section of the pre-cooling jacket is shaped to serve this purpose, and therefore is tapered outwardly from its communication with the second section of the pre-cooling jacket.
Also preferably, means are provided for creating and maintaining a vacuum in the space bounded by the casing, and enclosing the pre-cooling jacket and the cold shield.
The expansion of the gas fed to said tube through the nozzle effects rapid cooling of the space into which the gas expands and of the surfaces of said space. The gas, after so expanding, flows through the pre-cooling jacket and in heat exchange contact, towards a vent, in front of the device near the radiation transmitting means. Said heat exchange contact results in pre-cooling of the gas entering the tube.
The detector is mounted at one end of the cold shield in the vicinity of the outlet orifice of the cooler tube. It comprises a matrix of IR radiation detector elements and an electric circuit for receiving, elaborating and transmitting the signals generated by said elements. Preferably, a filter is provided on the top of the cold shield and the detector to filter out radiation that is not in the sensitivity range of the detector elements and may be considered as parasitic radiation with respect to IR detection. These components are per se part of the prior art and need not be particularly described.
However, preferably, the conductors that connect said electric circuit to the outside are located on the outer surface of the cold shield, lead from the end of the cold shield on which the detector is mounted to the end thereof on which the filter is mounted, and are electrically connected to a conductive component, that is close to the level of the optical window, through which the electrical signals issue from the device.